The invention relates generally to the field of idle speed controls. In particular, the invention relates to a control circuit for an idle speed controller for adjusting the idle speed of an internal combustion engine so as to conserve fuel during engine idle time, while offering greater than normal power when needed.
The fuel usage of an engine during idle, as at other times, is largely determined by engine speed. Engine speed influences the volumetric flow rate of the fuel-air mixture, and the internal friction and drag losses in the engine. Fuel usage at idle is a significant factor in fuel economy, both because fuel is expended without producing vehicle movement, and because an idle mixture must be a comparatively richer mixture of fuel to air to insure combustion, the mixing effect of higher engine speeds not being present, and the higher rotational momentum of higher engine speeds which tends to keep the engine in operation is not present.
Simply lowering the idle setting would conserve fuel, but with this there are certain conditions where the engine would be in danger of stalling. Such conditions include heavy electrical loads which are imposed on the engine through the generating system, as well as including the time when the engine has not yet achieved normal operating temperature after start-up.
A conventional internal combustion engine for an automobile has a throttle valve in an intake air passage of a carburetor, for control of the amount of air supply to the engine, the carburetor mixing fuel with the air in a predetermined ratio, the output power and rotational speed of the engine being controlled by the amount of fuel mixture per unit time. The throttle valve is opened and closed under the mechanical control of a throttle pedal, so that a driver may cause the vehicle to accelerate and decelerate. When the accelerator pedal is released, typically one or more springs cause the throttle plate to be moved to a substantially closed idle position. The movement of the throttle plate to its closed position is controlled by an idle stop, including a fixed stop member on the engine, and a moving stop member affixed to the throttle plate shaft, and generally containing an adjustment screw for cooperating with the fixed stop member to set the idle speed by regulating the position of the throttle plate.
Due to the large number of accessory loads in a modern automobile, together with the lack of manifold vacuum to support fuel mixture intake caused by pollution control accessories, it is often necessary to set the basic idle speed of an autombile internal combustion engine in excess of 1,000 RPM. This is necessary to insure that the engine will not stall when it is cold, will not stall when large electro-mechanical loads such as an air conditioner is imposed, when larger electrical loads such as fans and resistance heaters are imposed on the generating system, or when larger mechanical loads such as power steering are imposed on the engine at idle. Also, when the throttle plate is suddenly returned to its idle position, manifold vacuum increases sharply, causing a momentary variation of fuel mixture, the mixture becoming momentarily richer and unable to deliver optimum power. Then, the idle speed drops sharply, and the engine may stall if a high idle setting is not provided.
Various fast idle means operated by a bimetallic spring have been used to provide a cold idle stop position in conjunction with the operation of a choke valve in the carburetor, to provide a higher idle speed when the engine is cold, to compensate for poorer fuel vaporization and increased lubricating oil drag. Such cold idle devices are usually arranged so that the operator must press the throttle pedal to allow a cold idle stop member to be released after the engine has begun to warm up, so that, if the throttle pedal is not depressed, the engine will obtain an extremely high idle speed as it attains operating temperature, wasting fuel.
Various forms of servo mechanism systems have been proposed to avoid such deficiencies and maintain a substantially constant idle speed in the presence of varying engine temperature, intermittent accessory loads, and sudden throttle closing, especially where the mixture caused by a sudden throttle closing would result in emission components in excess of loads that can be successfully processed by emission control accessories.
Such servo mechanisms share a common deficiency of all mechanical servo mechanisms. The relatively complex structure necessary to achieve infinitely variable position of a mechanical element is prone to fail, and not suitable for extended, unmaintained use in a modern automobile. Also, such devices, in attempting to maintain a constant idle speed, do not allow the engine to idle at the lowest possible speed, as long as engine speed does not decrease below a predetermined minimum idle speed and do not provide for an increase in idle speed to allow a generator to provide sufficient output to meet a heavy electrical load. The instant invention overcomes the numerous deficiencies and problems of these previous approaches to controlling idle speed.